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Mammary Cancer

Julie Bulman‐Fleming, DVM, DACVIM (Oncology)

Veterinary Cancer Group, Tustin, CA, USA

Canine Mammary Cancer one mass. There is histologic and chronologic evidence
that mammary masses may progress through benign, in
Etiology/Pathophysiology situ, and noninvasive stages before developing into inva-
sive carcinoma. Estrogen and progesterone receptor lev-
Mammary tumors occur most commonly in intact els appear to decrease as tumors become larger, less
female dogs and in female dogs spayed later in life. differentiated, and more invasive.
Hormone exposure, primarily during development of
the mammary glands, is the most significant risk factor.
Increasing age and breed are also consistently reported Epidemiology
risk factors. Obesity at 9–12 months of age and diets Mammary gland tumors are the most common tumors
high in red meat have also been associated with increased in intact females, though the overall incidence depends
risk of tumor development. These factors may influence on the population in question. The lifetime risk of intact
the level of estrogen and progestin in the body or aug- female dogs has been reported to be as high as 26%. The
ment the effect of these hormones on mammary tissue. incidence in North America is lower than many other
Estrogens and progestins stimulate growth and devel- regions due to the more common practice of early ovari-
opment of mammary tissue and may contribute to tumo- ohysterectomy. Evaluation of several European registries
rigenesis through direct growth factor activity. Secondary showed that mammary tumors account for 50–70% of
increases in growth hormone and insulin‐like growth tumors in intact female dogs. Approximately 1% of mam-
factor 1 may also contribute. In humans, estrogen and mary tumors occur in males.
progestin also increase DNA and chromosomal damage.
Early ovariohysterectomy is protective against mam-
mary tumor development. Dogs spayed before their first Signalment
estrus have 0.5% lifetime risk. Dogs spayed before the sec- Mammary masses occur most frequently in older female
ond estrus have 8% risk and those spayed before the third dogs. The risk increases with age from 7 to 13 years, and
estrus 26% risk. The benefit of ovariohysterectomy beyond dogs with malignant masses tend to be older than those
the third heat appears slight, if any, though ovariohysterec- with benign tumors. Increased risk is reported for poodles,
tomy once a mammary mass has been diagnosed may be Chihuahuas, dachshunds, Yorkshire terriers, Maltese,
of some benefit. The age of first pregnancy and parity does cocker spaniels, English springer spaniels, Brittany
not appear to influence tumor development. Exposure to spaniels, German shepherd dogs, German shorthair
exogenous progestins and estrogens increases the risk, and pointers, English setters, and Doberman pinschers.
has also been linked to tumor development at younger age.
Hormone exposure was confirmed or suspected in most
male dogs diagnosed with mammary tumors. History and Clinical Signs
The entire mammary chain is exposed to the same Most dogs with mammary tumors are otherwise
hormones and growth factors, which is known as field healthy. Dogs usually present for a palpable mass or a
effect carcinogenesis. As such, dogs are at risk for multi- mammary mass may be an incidental finding. Careful
ple mammary masses. Up to 66% of dogs have more than palpation of the other glands, inguinal and axillary

Clinical Small Animal Internal Medicine Volume II, First Edition. Edited by David S. Bruyette.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/bruyette/clinical